1. Field of the Invention
The present invention relates to cables, in particular for low-voltage electrical energy distribution or for telecommunications, these cables having low-smoke self-extinguishing properties, and to the flame-retardant compositions used therein.
2. Description of the Related Art
Self-extinguishing cables can be produced having a flame-retardant coating made from a polymer composition to which fire-resistant properties have been given by adding a suitable additive. Polyolefin-based compositions based, for example, on polyethylene or ethylene/vinyl acetate copolymers, containing an organic halide combined with antimony trioxide as flame-retardant additive can, for example, be used for this purpose. However, halogenated flame-retardant additives have many drawbacks since they partially decompose during processing of the polymer, giving rise to halogenated gases that are toxic to workers and corrode metal parts of the polymer processing equipment. In addition, when they are placed directly in a flame, their combustion gives rise to large amounts of fumes containing toxic gases. Similar drawbacks are encountered when polyvinylchloride (PVC) supplemented with antimony trioxide is used as base polymer.
As reported, for example, by WO 99/05688, the production of self-extinguishing cables has been directed toward halogen-free compositions, using as flame-retardant filler inorganic oxides, preferably in hydrate or hydroxide form, in particular magnesium hydroxide or aluminium hydroxide.
Aluminium hydroxide starts to decompose at a relatively low temperature (about 190° C.), which can result in various drawbacks during extrusion of the polymer composition, with formation of bubbles and defects in the final product. Therefore, the use of aluminium hydroxide as flame retardant is generally limited to polymer materials which do not require high processing temperatures. In contrast, magnesium hydroxide has a decomposition temperature of about 340° C. and is characterized by greater heat stability and a high decomposition enthalpy. These properties make magnesium hydroxide particularly suitable as flame retardant filler in polymer compositions for coating cables, which require high extrusion temperatures and a small number of morphological defects.
In order to obtain an efficient flame-retardant effect, very large amounts of magnesium hydroxide must be added to the polymer material, generally about 120-250 parts by weight relative to 100 parts by weight of polymer material. Such high levels of magnesium hydroxide as filler lead to an increase of the polymer material viscosity and, as a consequence, to the lengthening of the manufacturing time. In addition, said viscosity increasing brings about a rising of the polymer material temperature during extrusion which, in turn, can cause the thermal degradation of the magnesium hydroxide contained therein.
High levels of magnesium hydroxide can also lead to a reduction in mechanical and elastic properties of the resulting polymer mixture, in particular as regards impact resistance, elongation and stress at break.
The reduction in mechanical and elastic properties of the resulting mixture is attributed to the low affinity of magnesium hydroxide with the polymer material.
Said affinity is connected to the magnesium hydroxide crystallinity and morphology, in term of geometric form and dimensional distribution of the magnesium hydroxide particles, beyond to the polarity of the surface and, in the case of natural magnesium hydroxide, to the impurities content, for example iron and manganese.
Therefore, research efforts have been directed towards modifying properties of magnesium hydroxide to improve its compatibility with the polymer matrix and its degree of purity.
For example, U.S. Pat. No. 6,676,920 B1 relates to a synthetic magnesium hydroxide particles having a hexagonal crystal form and having a specific aspect ratio (H) which is relatively large as compared with conventional ones. The range of the aspect ratio (H) is determined in correlation with values of an average secondary particle diameter (A), in the range of 0.15 to 5 μm, and a BET specific surface area (B), from 1 to 150 m2/g, of the magnesium hydroxide particles. The total content, as a metal content, of an iron compound content and a manganese compound content as impurities in the particles in the magnesium hydroxide particles is 0.01% by weight or less, preferably 0.005% by weight or less. The magnesium hydroxide particles are suitable for use as a flame retardant for synthetic resins.
The use of synthetic magnesium hydroxide as flame-retardant filler has a considerable impact on the cost of the finished product respect to the use of natural magnesium hydroxide obtained, for example, by grinding minerals such as brucite.
As from WO 99/05688, the magnesium hydroxide obtained by precipitation consists of flattened hexagonal crystallites that are substantially uniform both in size and morphology. In contrast, natural magnesium hydroxide has a highly irregular granular morphology in terms both of geometrical shape and of surface appearance.
Attempts have been made to improve the properties of natural magnesium hydroxide For example U.S. Pat. No. 5,474,602 describes improved fire retardant fillers for plastics material which consist of magnesium hydroxide particles of reduced average surface area. The particles are produced by contacting particles of a relatively high average surface area with an etching solution for a time sufficient to dissolve at least some of the particles and leave particles of reduced average surface area.
U.S. Pat. No. 6,025,424 relates to a flame retardant having heat deterioration resistance which is composed of magnesium hydroxide particles having (i) an average particle diameter of not more than 2 μm, (ii) a specific surface area, measured by a BET method, of not more than 20 m2/g and containing (iii) a total amount of an iron compound and a manganese compound of not more than 0.02% by weight in terms of metals.